How to use the roofing calculator
- Enter the length and width of the house footprint (the outside walls).
- Enter the eave overhang, which is how far the roof extends past the walls. It is added on all sides.
- Choose the roof pitch: the rise in inches for every 12 in of run (for example 6/12), or the angle in degrees in metric mode.
- Keep 10% waste for a simple roof, and use around 15% for roofs with valleys, hips and dormers.
- Check the shingle wrapper for bundles per square. Most shingles need 3 bundles per square.
The formulas
Footprint = (length + 2 × overhang) × (width + 2 × overhang) Pitch factor = √(1 + (rise ÷ 12)²) (metric: 1 ÷ cos(angle)) Roof surface = footprint × pitch factor Squares = roof surface × (1 + waste %) ÷ 100 Bundles = squares × bundles per square (rounded up)A roofing square is 100 sq ft of roof surface. The pitch factor converts flat footprint area into sloped roof area, and it gives the same answer for gable and hip roofs with a single pitch.
Pitch factors
| Pitch | Factor | Pitch | Factor |
|---|---|---|---|
| 3/12 | 1.031 | 8/12 | 1.202 |
| 4/12 | 1.054 | 9/12 | 1.250 |
| 5/12 | 1.083 | 10/12 | 1.302 |
| 6/12 | 1.118 | 12/12 | 1.414 |
Worked example
A 40 × 30 ft house with a 12 in overhang and a 6/12 roof:
- Footprint with overhang: 42 × 32 = 1,344 sq ft.
- Roof surface: 1,344 × 1.118 = 1,503 sq ft. With 10% waste: 1,653 sq ft, or 16.5 squares.
- Shingles: 16.53 × 3 = 49.6, so 50 bundles.
Tips
- Order ridge cap shingles, starter strips, underlayment and flashing separately.
- Working on a roof is dangerous. Use proper fall protection or hire a licensed roofer.
Frequently asked questions
What is a roofing square?
A roofing square is 100 square feet of roof surface. Roofers price and order materials by the square.
How many bundles of shingles are in a square?
Most asphalt shingles come 3 bundles to a square, so each bundle covers about 33 sq ft. Some heavier shingles need more, so check the wrapper.
How do I find my roof pitch?
Measure how many inches the roof rises over 12 inches of horizontal distance. A roof that rises 6 in over 12 in is a 6/12 pitch. In metric countries, pitch is usually given as an angle in degrees.
How much waste should I add for roofing?
Around 10% is common for simple gable roofs. Hip roofs, valleys and dormers create more cutting, so 15% or more is safer.